1. Field of the Invention
This invention relates to control systems of the type used, e.g., in process control to supply DC current signals, varying in a prescribed range, to a current loop including one or more load devices. More particularly, the invention relates to the detection of open circuit conditions in the load current loop.
2. Description of the Prior Art
Control systems which effect control through a DC current signal are widely known. The control current varies, for example, in a range of from 4 to 20 milliamperes to correspond to a variation of control from zero to 100 percent of scale. The DC current signal typically is applied serially to two or three final control elements. It has been found that in practice, open circuit conditions may occur in the output current loop.
Various arrangements for responding to such open circuit conditions are known. Upon the occurrence of an open circuit, the loop current falls below the minimum current value of, e.g., 4 milliamperes. In some prior art control systems, detection of an output current below 4 milliamperes causes an alarm to be generated. In most systems, however, an indication of an open condition is not supplied back to the controller, and the controller continues to send out control signals, which may result in undesirable effects on other control systems of a process. These effects are undesirable especially in large scale control systems. One prior art solution to this problem is to supply an open circuit detection signal back to the controller through an extra cable. However, the installation of such a cable is costly and not entirely satisfactory. Thus, none of the prior art control systems provides a simple and effective way of indicating to the controller that an open circuit load condition exists.